The present invention relates to a method of reading a flash memory device and, more particularly, to a method of reducing a read disturbance in a flash memory device.
Flash memory devices are widely used in electronic applications that need non-volatile memory devices. The flash memory cell has a one-transistor structure, which provides high density, high reliability and low current consumption. The flash memory devices are used in personal computers, personal digital assistants, digital cameras, cellular phones, etc. Also, program code, system data [e.g., basic input/output system (BIOS)] and firmware can be stored in the flash memory device. Flash memory devices, especially a NAND flash memory device, are more and more widely used because it can achieve high memory density with a relatively low cost.
As shown in FIG. 1, a NAND flash memory device includes a plurality of cell strings. A cell string 100 has a structure that includes a drain selection transistor 110, a plurality of cell transistors 131 to 134 and a source selection transistor which are connected in series. A drain of the drain selection transistor 110 is connected to a bit line BL and a source is connected to a drain of the cell transistor 131. A gate of the drain selection transistor 110 is connected to a drain selection line DSL. Gates of the cell transistors 131 to 134 are respectively connected to the word lines WL1 to WL4. A drain of the source selection transistor 120 is connected to a source of the cell transistor 134 and a source is connected to a common source line CSL. A gate of the source selection transistor 120 is connected to a source selection line SSL. Typical MOS transistors are used as the drain selection transistor 110 and the source selection transistor 120 and floating gate transistors are used as the cell transistors 131 to 133.
The cell transistors 131 to 133 are in an erase state or program state according to a threshold voltage of each cell transistor. The cell transistors have a relatively low voltage in the erase state, for example, a threshold voltage lower than 0V. Meanwhile, the cell transistors have a relatively high voltage in the program state, for example, a threshold voltage higher than 0V. A read operation, which needs to distinguish the state of a cell transistor, is done by applying a read voltage to the selected cell transistor and by distinguishing whether the selected cell transistor is turned on or off. For example, a voltage of 0V is applied to a word line of the selected transistor. That is, if the selected transistor is turned on, the selected transistor is in the erase state, because the threshold voltage is lower than the read voltage. Meanwhile, if the selected transistor is turned off, the selected transistor is in the program state, because the threshold voltage is higher than the read voltage.
However, when such a read operation is done repetitively, a read disturbance can occur. The read disturbance is a phenomenon where a threshold voltage of an erased cell transistor becomes abnormally high so that the transistor is incorrectly read in the program state. Several phenomena are suggested as a mechanism to cause such a read disturbance and one of them is caused by a hot carrier.
Specifically, as shown in FIG. 2, in the case of reading a cell transistor 132, a read voltage Vread of approximately a voltage of 0V is applied to a word line WL2 of the selected cell transistor 132 and a pass voltage Vpass higher than the read voltage Vread, (e.g., approximately 5V) is applied to unselected cells (e.g., the remaining word lines WL3 and WL4 of the remaining cell transistors 133 and 134). However, channel boosting by the pass voltage Vpass applied to the word line WL3 of the cell transistor 133 can occur. The cell transistor 133 is adjacent to the selected cell transistor 132 and thus a strong horizontal electric field and a strong vertical electric field are formed as indicated by arrows 241 and 242 in the drawing. As indicated by an arrow 243 in the drawing, electrons which come from an off-leakage current caused within a channel of the selected cell transistor 132 become hot carriers of high energy in the horizontal electric field and the vertical electric field and are injected into a floating gate FG of the adjacent cell transistor 133. Then, a threshold voltage of the adjacent cell transistor 133 increases and may be placed in an abnormal program state by the read disturbance phenomenon.